Generally, the performance of a polyester resin deteriorates gradually due to decomposition degradation of the polyester resin. In particular, since solar cell members, electric insulation members, and the like are placed under high temperature and high humidity for a long period of time, one major challenge is how to inhibit decomposition degradation for the polyester resin to be used and a film, a sheet, and an injection molded article using the resin.
The decomposition degradation of a polyester resin under high temperature and high humidity can be considered as being divided into thermal decomposition and hydrolysis.
First, decomposition of a polyester resin caused by heat is believed to proceed according to a reaction such as that described below. First, an ester linkage is cleaved by heat and, as a result, the polymer chain is cut and a carboxyl terminal group and a vinyl ester terminal are formed. Subsequently, a further reaction occurs at the vinyl ester terminal to eliminate acetaldehyde.
The hydrolysis due to water or water vapor is believed to proceed according to a reaction such as that described below. First, nucleophilic attack of a water molecule against an ester linkage causes cleavage of a polymer chain due to a hydrolysis reaction of the ester linkage to form a carboxyl group terminal and a hydroxy group terminal, and then the terminal hydroxy group formed causes back-biting and decomposition of the polymer chain is continued. In this reaction, a terminal carboxyl group plays a role to catalyze the hydrolysis reaction of a polymer and the hydrolysis is accelerated with increase of the amount of terminal carboxyl groups.
In order to inhibit such decomposition due to heat, water, or water vapor, many polyester resins small in the amount of terminal carboxyl groups have been proposed (see, for example, Patent Documents 1 and 2). The amount of terminal carboxyl groups has undoubtedly been reduced by such methods, but these are not sufficient for inhibiting decomposition for a long period of time. One of the reasons for this is that molding temperature increases due to a high melting point of a polyester resin. That is, even if the hydrolysis speed is reduced by blocking terminals of the polyester resin, new carboxyl groups are formed due to cleavage of a molecular chain caused by heat at the time of molding and, as a result, hydrolysis cannot be fully inhibited.
On the other hand, in order to solve such a decomposition degradation problem, there has been devised an invention to improve heat resistance by using a titanium compound and a phosphorus compound in combination (see, for example, Patent Document 3). When this method is used, a certain improvement in the heat resistance of a polymer is surely observed, but the level thereof is not necessarily high enough because a decomposition reaction is also promoted due to a high polymerization activity of the titanium compound. Moreover, there is a problem that the polymerization activity of titanium compound is lowered and, as a result, the degree of polymerization does not reach the goal or the polymerization time is remarkably prolonged and productivity worsens. Thus, excellent polymerization activity and sufficient heat resistance have not yet been achieved together.